First of all, welcome to TSS!
As for your question: oldies will only mate with other oldies. Do you have other Tamagotchis, or friends with Tamagotchis? Do any of them have Ojitchi on their Tamagotchis? If you can keep your Tamagotchi alive long enough to mate with an Ojitchi, you will be able to have another generation. You will also be met with a special secret character that you can only get by mating oldies.
If there is no chance of finding another Tamagotchi with an Ojitchi for your character to mate with, you'll just have to take care of your Tamagotchi as best as you can until she lives out her life. Then it will be time to reset and start a new egg.
Hope that helps!